Aluminum alloy magnetic disk substrates used in storage devices for computers and data centers are produced using substrates that have favorable plating properties and that are excellent in mechanical characteristics and workability. For example, the aluminum alloy magnetic disk substrates are produced from substrates based on an aluminum alloy according to JIS 5086 (including 3.5 mass % or more and 4.5 mass % or less of Mg, 0.50 mass % or less of Fe, 0.40 mass % or less of Si, 0.20 mass % or more and 0.70 mass % or less of Mn, 0.05 mass % or more and 0.25 mass % or less of Cr, 0.10 mass % or less of Cu, 0.15 mass % or less of Ti, and 0.25 mass % or less of Zn with the balance of Al and unavoidable impurities).
Common magnetic disks made of aluminum alloys are produced by first producing an annular aluminum alloy substrate, plating the aluminum alloy substrate, and then depositing a magnetic substance on a surface of the aluminum alloy substrate.
For example, a magnetic disk made of an aluminum alloy, including the MS 5086 alloy, is produced by the following production steps. First, an aluminum alloy allowed to contain desired chemical components is cast to obtain an ingot, and the ingot is subjected to homogenization treatment, then to hot rolling, and then to cold rolling, to produce a rolled material having a thickness required for the magnetic disk. It is preferable to anneal the rolled material in the cold rolling and/or the like as needed. Then, the rolled material is stamped to obtain aluminum alloy sheets having an annular shape. In order to eliminate distortion and/or the like occurring in the previous production steps, the aluminum alloy sheets having the annular shape obtained by stamping the rolled material are layered, and the resultant is subjected to pressurization annealing in which the resultant is flattened by annealing the resultant while pressurizing both top and under surfaces of the resultant. As a result, an annular aluminum alloy disk blank is produced.
The disk blank produced in such a manner is subjected in turn to cutting work, grinding work, degreasing treatment, etching treatment, desmutting treatment, and zincate treatment (Zn substitution treatment) as pretreatment. Then, the disk blank is subjected to electroless plating with Ni—P which is a rigid non-magnetic metal as undercoat treatment, and a plated surface of the disk blank is flattened by polishing, followed by sputtering a magnetic substance, to produce the magnetic disk made of an aluminum alloy.
In recent years, the environment enveloping HDDs has been in the process of great change. The larger capacities, higher densities, and, in addition, speedup of the HDDs have become necessary for the larger storage capacities of data centers, caused by development of cloud service, and for competition with SSDs which have been new storage devices. For the large capacities of the HDDs, it is effective to increase the number of magnetic disks placed in a storage device. Thus, thinned aluminum alloy base materials for magnetic disks have been demanded. However, the thinned aluminum alloy base materials for magnetic disks result in a decrease in rigidity, and in an increase in exciting force due to an increase in fluid force in high-speed rotation caused by the speedup, thereby causing a problem of occurrence of disk flutter. The disk flutter occurs because high-speed rotation of magnetic disks causes unstable airflow to be generated between the disks, and the airflow results in vibration (fluttering) of the magnetic disks. The disk flutter is considered to occur because the low rigidity of an aluminum alloy base material results in the increased vibration of the magnetic disks, and a head which is a reader is incapable of following such a variation. The occurrence of fluttering results in an increase in the positioning error of the head. Therefore, reduction in disk flutter has been earnestly demanded.
For the larger capacities of the HDDs, it is also effective to increase a storage capacity per magnetic disk. The presence of defects such as pits on an electroless Ni—P-plated surface results in the necessity of reading and writing data in portions other than the peripheries of the defects. As a result, a storage capacity per magnetic disk is decreased in proportion to the number of the defects. As described above, a decrease in defects on an electroless Ni—P-plated surface is essential for increasing a storage capacity.
In light of such actual circumstances, an aluminum alloy magnetic disk substrate having both characteristics of reducing disk flutter and decreasing defects on an electroless Ni—P plated surface has been earnestly desired in recent times. For example, a conventionally used Al—Mg-based alloy according to JIS 5086 or the like is incapable of achieving a decrease in disk flutter. For decreasing the disk flutter, it is effective to distribute a compound in an aluminum alloy, and therefore, it is necessary to examine an alloy type that has not been examined until now. However, since the number of defects on an electroless Ni—P plated surface is increased with increasing the amount of a compound in an aluminum alloy, measures to reduce the contents of Fe and Si have been taken for conventional aluminum alloy base materials. For satisfying both the characteristics of reducing disk flutter and decreasing defects on an electroless Ni—P plated surface, it is necessary to solve the two contradictory problems.
For example, Patent Literature 1 discloses the composition of an aluminum alloy base material to which a large amount of Si is added in order to reduce disk flutter. Patent Literature 2 discloses a technology of suppressing plating defects by adding sulfate ions to washing water in an electroless Ni—P plating step.
In the aluminum alloy substrate disclosed in Patent Literature 1, however, grinding work is precluded because a large amount of Si is added to the aluminum alloy substrate, and, in addition, it is difficult to remove Si on a surface of the aluminum alloy substrate, whereby it is impossible to solve a problem in that the number of defects on an electroless Ni—P plated surface is increased. The technology of Patent Literature 2 is a technology that exhibits an effect in the case of an aluminum alloy substrate with a small amount of a compound, and therefore, the effect is unexpectable when a compound is dispersed to reduce disk flutter.